A typical liquid supply system includes a portable liquid supply tank to hold a liquid and a pump to supply the liquid in the supply tank to a reservoir tank in response to a signal from a sensor attached to the reservoir tank. The pump, operating in response to a signal from the sensor, supplies liquid thereby increasing the volume of liquid in the reservoir tank from a first predetermined volume to a second predetermined volume (the second predetermined volume being greater than the first predetermined volume). In other words, when the volume of liquid in the reservoir tank drops to a first predetermined volume, the sensor attached to the reservoir tank outputs a signal which causes the pump to supply liquid from the supply tank to the reservoir tank until the volume of liquid in the reservoir tank rises a second predetermined volume greater. Typically, the volume of liquid in the reservoir tank is detected by detecting the position of a float.
The typical liquid supply system also includes a device for preventing the no-load operation of the pump when the liquid in the supply tank is exhausted. A prior art device for preventing the no-load operation of a pump is disclosed in Japanese Utility Model Application Laid-open Gazette No. 52-116710. Such a device for preventing the no-load operation of a pump as disclosed by this reference includes a sensor for detecting the amount of liquid remaining in the supply tank. The sensor signals the pump stopping the supply of liquid to the reservoir tank when the liquid volume in the supply tank drops below a predetermined volume.
However, the pump is signaled to stop not only when the device for preventing no-load operation determines the amount of liquid in the supply tank has fallen below a predetermined volume but also when, in normal operation, the amount of liquid supplied to the reservoir tank is sensed to be sufficient. Accordingly, an operator or a user cannot correctly know from stoppage of the pump's operation whether or not it is necessary to replenish the supply of liquid in the supply tank because an insufficient amount of liquid remains for proper operation of the liquid supply system.
Additionally, if a connecting mechanism, for example, a sealing coupler, disposed on the fluid circuit between the supply tank and the pump happens to be incompletely or improperly connected such that the liquid flow from the supply tank is interrupted, the pump will operate, continuously, attempting to supply liquid to the reservoir tank, even though there may be sufficient liquid in the supply tank to adequately supply the reservoir tank. The prior art device for preventing no-load operation of a pump disclosed in Japanese Utility Model Application Laid-open Gazette No. 52-116710 does not provide an indication to a user of any abnormal conditions which may be experienced by the liquid supply apparatus such as incomplete connection of the connecting mechanism causing interrupted liquid flow.